There are times when you have a chunk of text that you want to do something with, e.g. replace something on it, or quickly edit it.
While you can open your favorite non-RubyMine editor for this, there is also a plugin:.
It allows RubyMine to open temporary files (actually they are saved, but somewhere inside the plugin's directory) so you don't need to switch to a text editor like gEdit that works differently and may not even offer what you are used to.
Note that RubyMine also offers something similar as a built-in feature: The "Tools" Menu holds an action "Create New Scratch File" that will also create a new file that does not live inside of your project directory. However, reopening those is a bit awkward (in your project tree, use the dropdown in the top left), and has no cool keyboard shortcuts. So you might want to use the Scratch plugin instead.
Inside the "Tools" menu you will find a submenu "Scratch". Or use the keyboard shortcuts:
Syntax highlighting etc work depending on the file extension, so you may want to add a new
.rb file that will give you Ruby syntax highlighting.